Remove `Amount::MAX` and replace it with the value in `MAX_MONEY`

[tcharding]

#3688 is quite a convincing argument for limiting the value of Amount to MAX_MONEY. Also we have #620, which I've read twice in the past and don't want to read again because its so long but I think if we do the following we can close both issues

• Make Amount::MAX equal to Amount::MAX_MONEY
• Deprecate/remove max_money ?
• Enforce the invariant in all addition and multiplication functions

If someone has the patience to read through #620 again and check I haven't forgotten anything that would be great.

[jirijakes]

Enforce the invariant in all addition and multiplication functions

Can this be done independently from adding something like UncheckedAmount at the same time? In other words, shouldn't this task (making MAX MAX_MONEY) be part of a broader task to split Amount into two (one enforcing invariants, one not)? Or does this task assume using i64 for arithmetic operations that may exceed 21M?

[yancyribbens]

@jirijakes

This was discussed here: #620 where we discussed that very question.

[jirijakes]

This was discussed here: #620 where we discussed that very question.

Without a conclusion, right? So I am wondering whether the point I quoted (enforce 21M invariant in Amount) assumes anything about that or not. In other words, after this task is closed as finished, what shall I do if I need to add bitcoin amounts together when I expect them to exceed 21M?

[tcharding]

Convert them to plain old u64 and add them? Or said another way, what does adding UncheckedAmount provide if the type is not used anywhere in the rust-bitcoin API?

[jirijakes]

Convert them to plain old u64 and add them?

Thanks, that answers the question I had.

Or said another way, what does adding UncheckedAmount provide if the type is not used anywhere in the rust-bitcoin API?

I think there are some benefits but this issue is not the place to discuss them.

[apoelstra]

Cool, agreed with all three suggestions in the OP.

Also agreed that UncheckedAmount does not have a clear benefit over u64 and that it's definitely not needed for the MAX_MONEY thing, since users can always just use u64 (or u128 or whatever).